Device and method for intra-prediction of a prediction block of a video image

ABSTRACT

A device is configured to select a directional intra-prediction mode from a set of directional intra-prediction modes, wherein each directional intra-prediction mode corresponds to a different intra-prediction angle. Further, the device is configured to select a filter from a set of filters based on the selected directional intra-prediction mode. Further, the device is configured to determine, for a given prediction sample of the prediction block, a reference sample from a set of reference samples based on the selected directional intra-prediction mode, and apply the selected filter to the determined reference sample.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No.PCT/RU2018/000430, filed on Jun. 29, 2018, the disclosure of which ishereby incorporated by reference in its entirety.

TECHNICAL FIELD

Embodiments of the present invention relate to the field of pictureprocessing, for example still picture/image and/or video picture/imagecoding. In particular, embodiments relate to a device forintra-prediction, namely for intra-predicting a prediction block of avideo image. The device may be or be part of a video image encoder or avideo image decoder. The device is particularly configured to performdirectional intra-prediction of the prediction block. Embodiments alsorelate also to a corresponding intra-prediction method.

BACKGROUND

Video coding (video encoding and decoding) is used in a wide range ofdigital video applications, for example, broadcast digital TV, videotransmission over internet and mobile networks, real-time conversationalapplications such as video chat, video conferencing, DVD and Blu-raydiscs, video content acquisition and editing systems, and camcorders ofsecurity applications.

Since the development of the block-based hybrid video coding approach inthe H.261 standard in 1990, new video coding techniques and tools weredeveloped and formed the basis for new video coding standards. One ofthe goals of most of the video coding standards was to achieve a bitratereduction compared to its predecessor without sacrificing picturequality. Further video coding standards comprise MPEG-1 video, MPEG-2video, ITU-T H.262/MPEG-2, ITU-T H.263, ITU-T H.264/MPEG-4, Part 10,Advanced Video Coding (AVC), ITU-T H.265, High Efficiency Video Coding(HEVC), and extensions, e.g., scalability and/or three-dimensional (3D)extensions, of these standards.

Video compression is able to achieve the desired bitrate reduction, butis a complex task. In particular, video compression is constrained bytwo contradicting parameters: compression efficiency and computationalcomplexity. Video coding standards, such as ITU-T H.264/AVC or ITU-TH.265/HEVC, provide a good tradeoff between these parameters. For thatreason, support of video coding standards is a mandatory requirement foralmost any video compression application.

The state-of-the-art video coding standards are based on partitioning ofa source picture into blocks. Processing of these blocks depend on theirsize, spatial position and a coding mode specified by an encoder.

Coding modes can be classified into two groups according to the type ofprediction: intra- and inter-prediction modes. Intra-prediction modesuse pixels of the same picture to generate reference samples tocalculate the prediction values for the pixels of the block beingreconstructed. Intra-prediction is also referred to as spatialprediction. Inter-prediction modes are designed for temporal prediction,and use reference samples of previous or next pictures to predict pixelsof the block of the current picture.

After a prediction stage, transform coding is performed for a predictionerror that is the difference between an original signal and itsprediction. Then, the transform coefficients and side information areencoded using an entropy coder (e.g., CABAC for AVC/H.264 andHEVC/H.265). The recently adopted ITU-T H.265/HEVC standard (ISO/IEC23008-2:2013, “Information technology—High efficiency coding and mediadelivery in heterogeneous environments—Part 2: High efficiency videocoding”, November 2013) declares a set of state-of-the-art video codingtools that provide a reasonable tradeoff between coding efficiency andcomputational complexity. An overview on the ITU-T H.265/HEVC standardhas been given by Gary J. Sullivan, “Overview of the High EfficiencyVideo Coding (HEVC) Standard”, in IEEE Transactions on Circuits andSystems for Video Technology, Vol. 22, No. 12, December 2012, the entirecontent of which is incorporated herein by reference.

Similarly to the ITU-T H.264/AVC video coding standard, the HEVC/H.265video coding standard provides for a division of the source picture intoblocks, e.g., Coding Units (CUs). Each of the CUs can be further splitinto either smaller CUs or Prediction Units (PUs). A PU can be intra- orinter-predicted according to the type of processing applied for thepixels of the PU. In case of inter-prediction, a PU represents an areaof pixels that is processed by motion compensation using a motion vectorspecified for a PU. For intra-prediction, the adjacent pixels ofneighbor blocks are used as reference sample to predict a current block.

A PU specifies a prediction mode that is selected from the set ofintra-prediction modes for all the Transform Units (TUs) contained inthis PU. That is, the intra-prediction mode is the same for each TU ofthe PU. A TU can have different sizes (e.g., 4×4, 8×8, 16×16 and 32×32pixels) and can be processed in different ways. For a TU, transformcoding is performed, i.e. the prediction error is transformed with adiscrete cosine transform or a discrete sine transform (in theHEVC/H.265 standard, it is applied to intra-coded blocks) and quantized.Hence, reconstructed pixels contain quantization noise (it can becomeapparent, for examples, as blockiness between units, ringing artifactsalong with sharp edges, etc.) that in-loop filters such as DBF, SAO andALF try to suppress. The use of sophisticated prediction coding (such asmotion compensation and intra-prediction) and partitioning techniques(e.g., Quad-Tree (QT) for CUs and PUs as well as Residual Quad-Tree(RQT) for TUs in the HEVC/H.265 standard and Quad-Tree and Binary Tree(QTBT) for the Joint Exploration Model (JEM) reference software startingfrom version JEM-3.0) allowed the standardization committee tosignificantly reduce the redundancy in PUs. The fundamental differencebetween the QT and QTBT partitioning mechanisms is that the latter oneenables not only square but also rectangular blocks by usingpartitioning based on both quad- and binary tree. The present inventionrelates to directional intra-prediction and introduces new modificationof directional intra-prediction modes.

According to the HEVC/H.265 standard, 35 intra-prediction modes areavailable. As shown in FIG. 8 this set contains the following modes:

Planar mode (the intra-prediction mode index is 0),

DC mode (the intra-prediction mode index is 1),

Directional modes (the range of the intra-prediction mode index valuesis from 2 to 34) shown by solid arrows in FIG. 8. The set of directionalintra-prediction modes was extended up to 65 modes (i.e. almost doubled)by decreasing an angular step between directional intra-prediction modesby a factor of 2. These additional modes are shown by the dashed arrowsin FIG. 8.

For the JEM-3.0 software, the new partitioning mechanism known as QTBTwas proposed. As depicted in FIG. 9, QTBT partitioning can provide notjust square but rectangular blocks. Of course, some signaling overheadand increased computational complexity at the encoder side are the priceof the QTBT partitioning, as compared to conventional QT basedpartitioning used e.g., in the HEVC/H.265 standard. Nevertheless, theQTBT-based partitioning is endowed with better segmentation propertiesand, hence, demonstrates significantly higher coding efficiency than theconventional QT.

However, when introducing QTBT the set of available directionalintra-prediction modes was not changed accordingly. In particular,asymmetry of rectangular blocks was not taken into account, as shown inFIG. 10. Thus, the same number of reference samples are used along boththe shorter and the longer sides of rectangular blocks. In the currentimplementation of the QTBT framework, the number of directionalintra-prediction modes depends on neither aspect ratio of blocks, nor onthe actual availability of reference samples. As a result, there arehighly improbable reference samples used for the shorter side of therectangular block, while there are probable reference samples that arenot used for the longer side.

Notably, as shown in FIG. 11, in this document, the terms of “verticallyoriented block” (“vertical orientation of a block”) and “horizontallyoriented block” (“horizontal orientation of a block”) are applied torectangular blocks generated by the QTBT framework. FIG. 11 showsparticularly (a) a horizontally oriented block, and (b) a verticallyoriented block.

In the contribution JVET-D0113, it was further proposed to apply amechanism, wherein the number of directional intra-prediction modes isadjustable. In particular, it was proposed to further increase thenumber of directional intra-prediction modes to 131 for large blockssizes, while reducing the number of directional intra-prediction modesfor small block sizes. The switching of the number of directionalintra-prediction modes based on block sizes is controlled by twothreshold values, which are signaled in SPS as log 2 values minus 4 andminus 6, respectively. The first threshold indicates the largest blocksize that can have 35 intra-prediction mode directions, and the secondthreshold indicates the largest block size that has 67 intra-predictionmode directions, all other blocks use 131 intra-prediction modedirections. In the default setting, thresholds are signaled as 4 and 6,respectively, and for higher resolution pictures it is set to 5 and 8.

In the implementation, a directional intra-prediction mode index isalways represented by a 131 mode range regardless of the number ofdirectional intra-prediction modes actually used. For 67intra-prediction modes being actually used, only every second angular(directional) mode is allowed, and for 35 modes, only every fourthangular (directional) mode is allowed. Therefore, duringintra-prediction mode signaling, an intra-prediction mode of aneighboring block may need to be rounded towards the nearest, second, orfourth angular intra-prediction mode, if the current block uses smallerthan 131 intra-prediction mode directions, as is explained in FIG. 12.This conversion is done by applying right and left shifts of 1 or 2 toan intra-prediction mode. If the mode is not MPM, the mode signaling isfollowing the same process as in JEM-3.0, but with a different number ofintra-prediction modes. Planar and DC modes are kept unchanged, and donot require mode conversion. To accommodate the increased number ofintra-prediction modes, 4-tap intra filters are extended from 1/32 to1/64 fractional pel.

Further, a technology was proposed recently to address the problem ofhow many directional intra-prediction modes should be included into anintra-prediction mode set for a rectangular block. As shown in FIG. 13,according to the proposed technology, the set of directionalintra-prediction modes can be extended subject to the aspect ratio of aprediction block, and can be signaled by mapping the added directionalintra-prediction modes to the conventional subset.

FIG. 14 illustrates in this respect a case of an intra-prediction in adiagonal direction with an angle equal to 45° associated with adirectional intra-prediction mode. Corresponding HEVC intra mode indexesfor this case are 2 (from bottom-left) and 35 (from upper-right).

However, if a similar intra-prediction mechanism is applied to anglessmaller than 45°, i.e. for the extended directional intra-predictionmodes, the situation is as shown in FIG. 15. Namely, when anintra-prediction direction is specified to be acute (i.e. less than 45°)apparent discontinuities can be observed in the prediction. The sourceof these discontinuities is particularly that the difference betweenreference samples positions between two adjacent rows of predictionsamples may become larger than one reference sample. This problemrelates to methods of reference samples processing and performingintra-prediction interpolation.

SUMMARY

In view of the above-mentioned implementations, the present inventionaims to further improve hybrid video coding. In particular, theinvention has the objective to provide a device and method for animproved intra-prediction of a prediction block of a video image. Theinvention particularly aims for additional coding gain without addinghardware and computational complexity. Specifically, the inventionintends to overcome the above-described issue that occurs at acuteangles of less than 45°, i.e. it wants to suppress discontinuities atthese acute angles. The invention should be easily implemented in codecsthat use conventional directional intra-prediction mechanisms.

The objective of the present invention is solved according toembodiments of the invention defined by the features of the independentclaims. Further advantageous implementations of the embodiments aredefined by the features of the dependent claims.

In particular, the invention proposes reducing the discontinuities bychanging angular steps (step angles) between neighboring angles ofdirectional intra-prediction modes in at least a subset of thedirectional intra-prediction modes, e.g., for intra-prediction modesassociated with acute intra-prediction angles. This solution isapplicable mainly to rectangular blocks produced by such partitioningframeworks as QTBT and MTT.

A first aspect of the invention provides a device for intra-predicting aprediction block of a video image, the device being configured to selecta directional intra-prediction mode from a set of directionalintra-prediction modes, wherein each directional intra-prediction modecorresponds to a different intra-prediction angle, determine, for agiven prediction sample of the prediction block, a reference sample froma set of reference samples based on the selected directionalintra-prediction mode, and apply a filter to the determined referencesample, wherein, within a subset of the directional intra-predictionmodes, an angular step between the directional intra-prediction modes isdefined in dependence of a filter length of the filter.

The device according to the first aspect provides the followingadvantages:

Additional coding gain can be reached.

It can be used in many potential applications in hybrid video codingparadigms that are compatible with the HM software and the VPX videocodec family as well as the JEM and VTM software and the VPX/AV1 videocodec family that are a state-of-the-art and a next-generation videocoding frameworks, respectively.

Hardware and computational complexities are kept low.

The device can be easily implemented in such codecs that useconventional directional intra-prediction mechanisms.

By defining the angular step based on the filter length of the appliedfilter, particularly by selecting a smaller angular step for a shorterfilter length, the angles associated with the different directionalintra-prediction modes in the subset span in total a smaller angularrange. Thus, it can be ensured that even for the most acuteintra-prediction angle, discontinuities are suppressed due to thefilter.

Notably, the prediction block may be a TU or a PU. The device isconfigured to process, as described for the given prediction sample,each prediction sample in the prediction block. Thus, the device isconfigured to perform intra-prediction of the entire prediction block inthe video image. A sample is an intersection of a channel and a pixel inthe video image. For instance, each pixel of the video image may includethree samples for Red, Green and Blue.

In an implementation form of the first aspect, the angular step islarger for a larger filter length of the filer and smaller for a smallerfilter length of the filter.

For a larger filter length, more acute intra-prediction angles can beused for improving the intra-prediction results, without causingdiscontinuities. The angles associated with the directionalintra-prediction modes in the subset can span in total a larger angularrange.

In a further implementation form of the first aspect, the angular stepis between 2-4 degrees, particularly is 2, 2.5, 3 or 4 degrees,depending on the filter length of the filter.

In a further implementation form of the first aspect, the angular stepis denoted Δα and is defined by

${\Delta\alpha} = \frac{\arctan N_{f}}{\Delta M_{0}}$

wherein N_(f) is the filter length of the filter and ΔM₀ is the numberof directional intra-prediction modes in the subset.

In this way, the angular step is optimally defined based on the filterlength to obtain the best results. Notably, the number of directionalintra-prediction modes in the subset may depend, for instance, on theaspect ratio of the prediction block. That is, the subset of directionalintra-prediction modes is selected based on the aspect ratio of theprediction block.

In a further implementation form of the first aspect, the filter lengthof the filter is selected in dependence of the aspect ratio of theprediction block.

For instance, the larger the aspect ratio, the more acute theintra-prediction angles may become at the longer side of the rectangularblock. Accordingly, a larger filter length may be required. The largerthe filter length, the larger also the angular step in the subset.

In a further implementation form of the first aspect, the filter lengthof the filter is selected in dependence of a height and/or a width ofthe prediction block.

In a further implementation form of the first aspect, the filterperforms a smoothing over the determined reference sample and one ormore adjacent reference samples according to the filter length, whenapplied to the determined reference sample.

In a further implementation form of the first aspect, the referencesamples of the set of reference samples are arranged in a row of thevideo image adjacently above and above-right the prediction block,and/or are arranged in a column of the video image adjacently left andleft-under the prediction block.

In a further implementation form of the first aspect, the device isconfigured for encoding and/or decoding the video image, or the deviceis a video encoder and/or video decoder.

For instance, the device of the first aspect can be included in or be anintra-prediction unit of an encoder or decoder.

A second aspect of the invention provides for intra-predicting aprediction block of a video image, the method comprising: selecting adirectional intra-prediction mode from a set of directionalintra-prediction modes, wherein each directional intra-prediction modecorresponds to a different intra-prediction angle, determining, for agiven prediction sample of the prediction block, a reference sample froma set of reference samples based on the selected directionalintra-prediction mode, and applying a filter to the determined referencesample, wherein, within a subset of the directional intra-predictionmodes, an angular step between the directional intra-prediction modes isdefined in dependence of a filter length of the filter.

In an implementation form of the second aspect, the angular step islarger for a larger filter length of the filer and smaller for a smallerfilter length of the filter.

In a further implementation form of the second aspect, the angular stepis between 2-4 degrees, particularly is 2, 2.5, 3 or 4 degrees,depending on the filter length of the filter.

In a further implementation form of the second aspect, the angular stepis denoted Δα and is defined by

${\Delta\alpha} = \frac{\arctan N_{f}}{\Delta M_{0}}$

wherein N_(f) is the filter length of the filter and ΔM₀ is the numberof directional intra-prediction modes in the subset.

In a further implementation form of the second aspect, the filter lengthof the filter is selected in dependence of the aspect ratio of theprediction block.

In a further implementation form of the second aspect, the filter lengthof the filter is selected in dependence of a height and/or a width ofthe prediction block.

In a further implementation form of the second aspect, the filterperforms a smoothing over the determined reference sample and one ormore adjacent reference samples according to the filter length, whenapplied to the determined reference sample.

In a further implementation form of the second aspect, the referencesamples of the set of reference samples are arranged in a row of thevideo image adjacently above and above-right the prediction block,and/or are arranged in a column of the video image adjacently left andleft-under the prediction block.

In a further implementation form of the second aspect, the method is forencoding and/or decoding the video image, or the method is performed bya video encoder and/or video decoder.

The method of the second aspect and its implementation forms achieve thesame advantages and effects described above for the device of the firstaspect and its respective implementation forms.

It has to be noted that all devices, elements, units and means describedin the present application could be implemented in the software orhardware elements or any kind of combination thereof. All steps whichare performed by the various entities described in the presentapplication as well as the functionalities described to be performed bythe various entities are intended to mean that the respective entity isadapted to or configured to perform the respective steps andfunctionalities. Even if, in the following description of specificembodiments, a specific functionality or step to be performed byexternal entities is not reflected in the description of a specificdetailed element of that entity which performs that specific step orfunctionality, it should be clear for a skilled person that thesemethods and functionalities can be implemented in respective software orhardware elements, or any kind of combination thereof.

Details of one or more embodiments are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages will be apparent from the description, drawings, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following embodiments of the invention are described in moredetail with reference to the attached figures and drawings, in which:

FIG. 1 is a block diagram showing an example structure of a videoencoder configured to implement embodiments of the invention.

FIG. 2 is a block diagram showing an example structure of a videodecoder configured to implement embodiments of the invention.

FIG. 3 is a block diagram showing an example of a video coding systemconfigured to implement embodiments of the invention.

FIG. 4 shows (a) a source of discontinuities for a case whenintra-prediction angles are smaller than 45°, and (b) inter-rowdiscontinuities for the case when intra-prediction angles are smallerthan 45°.

FIG. 5 is a block diagram showing a device according to an embodiment ofthe invention.

FIG. 6 shows the concept of using different angular steps for extendeddirectional intra-prediction modes.

FIG. 7 shows a flow-diagram of a method according to an embodiment ofthe invention.

FIG. 8 shows intra-prediction modes in the HM and JEM software (theangular/directional modes are marked by dashed lines are introduced justfor JEM but not for HM).

FIG. 9 shows schematically a QTBT partitioning.

FIG. 10 shows a current implementation of a directional intra-predictionmechanism in the QT and QTBT frameworks.

FIG. 11 explains an orientation of rectangular blocks, particularlyshows a rectangular block with (a) a horizontal orientation and (b) avertical orientation.

FIG. 12 shows intra-mode selection proposed in JVET-D0113.

FIG. 13 shows a proposed extension of directional intra-predictionmodes.

FIG. 14 shows schematically a distance between reference samples forintra-predicting two adjacent rows of prediction samples forintra-prediction angles equal to 45°.

FIG. 15 shows schematically a distance between reference samples forintra-prediction two adjacent rows of prediction samples forintra-prediction angles smaller than 45°.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

In the following description, reference is made to the accompanyingfigures, which form part of the disclosure, and which show, by way ofillustration, specific aspects of embodiments of the invention orspecific aspects in which embodiments of the present invention may beused. It is understood that embodiments of the invention may be used inother aspects and comprise structural or logical changes not depicted inthe figures. The following detailed description, therefore, is not to betaken in a limiting sense, and the scope of the present invention isdefined by the appended claims.

For instance, it is understood that a disclosure in connection with adescribed method may also hold true for a corresponding device or systemconfigured to perform the method and vice versa. For example, if one ora plurality of specific method steps are described, a correspondingdevice may include one or a plurality of units, e.g., functional units,to perform the described one or plurality of method steps (e.g., oneunit performing the one or plurality of steps, or a plurality of unitseach performing one or more of the plurality of steps), even if such oneor more units are not explicitly described or illustrated in thefigures. On the other hand, for example, if a specific apparatus isdescribed based on one or a plurality of units, e.g., functional units,a corresponding method may include one step to perform the functionalityof the one or plurality of units (e.g., one step performing thefunctionality of the one or plurality of units, or a plurality of stepseach performing the functionality of one or more of the plurality ofunits), even if such one or plurality of steps are not explicitlydescribed or illustrated in the figures. Further, it is understood thatthe features of the various exemplary embodiments and/or aspectsdescribed herein may be combined with each other, unless specificallynoted otherwise.

Video coding typically refers to the processing of a sequence ofpictures, which form the video or video sequence. Instead of the termpicture the terms frame or image may be used as synonyms in the field ofvideo coding. Video coding comprises two parts, video encoding and videodecoding. Video encoding is performed at the source side, typicallycomprising processing (e.g., by compression) the original video picturesto reduce the amount of data required for representing the videopictures (for more efficient storage and/or transmission). Videodecoding is performed at the destination side and typically comprisesthe inverse processing compared to the encoder to reconstruct the videopictures. Embodiments referring to “coding” of video pictures (or videoimages or pictures in general, as will be explained later) shall beunderstood to relate to both, “encoding” and “decoding” of videopictures. The combination of the encoding part and the decoding part isalso referred to as CODEC (COding and DECoding).

In case of lossless video coding, the original video pictures can bereconstructed, i.e. the reconstructed video pictures have the samequality as the original video pictures (assuming no transmission loss orother data loss during storage or transmission). In case of lossy videocoding, further compression, e.g., by quantization, is performed, toreduce the amount of data representing the video pictures, which cannotbe completely reconstructed at the decoder, i.e. the quality of thereconstructed video pictures is lower or worse compared to the qualityof the original video pictures.

Several video coding standards since H.261 belong to the group of “lossyhybrid video codecs” (i.e. combine spatial and temporal prediction inthe sample domain and 2D transform coding for applying quantization inthe transform domain). Each picture of a video sequence is typicallypartitioned into a set of non-overlapping blocks and the coding istypically performed on a block level. In other words, at the encoder thevideo is typically processed, i.e. encoded, on a block (video block)level, e.g., by using spatial (intra picture) prediction and temporal(inter picture) prediction to generate a prediction block, subtractingthe prediction block from the current block (block currentlyprocessed/to be processed) to obtain a residual block, transforming theresidual block and quantizing the residual block in the transform domainto reduce the amount of data to be transmitted (compression), whereas atthe decoder the inverse processing compared to the encoder is applied tothe encoded or compressed block to reconstruct the current block forrepresentation. Furthermore, the encoder duplicates the decoderprocessing loop such that both will generate identical predictions(e.g., intra- and inter predictions) and/or re-constructions forprocessing, i.e. coding, the subsequent blocks.

As video picture processing (also referred to as moving pictureprocessing) and still picture processing (the term processing comprisingcoding), share many concepts and technologies or tools, in the followingthe term “picture” is used to refer to a video picture of a videosequence (as explained above) and/or to a still picture to avoidunnecessary repetitions and distinctions between video pictures andstill pictures, where not necessary. In case the description refers tostill pictures (or still images) only, the term “still picture” shall beused.

In the following an encoder 100, a decoder 200 and a coding system 300for implementing embodiments of the invention are described based onFIGS. 1 to 3, before describing the embodiments of the invention in moredetail based on FIGS. 4 to 11.

FIG. 3 is a conceptional or schematic block diagram illustrating anembodiment of a coding system 300, e.g., a picture coding system 300,wherein the coding system 300 comprises a source device 310 configuredto provide encoded data 330, e.g., an encoded picture 330, e.g., to adestination device 320 for decoding the encoded data 330.

The source device 310 comprises an encoder 100 or encoding unit 100, andmay additionally, i.e. optionally, comprise a picture source 312, apre-processing unit 314, e.g., a picture pre-processing unit 314, and acommunication interface or communication unit 318.

The picture source 312 may comprise or be any kind of picture capturingdevice, for example for capturing a real-world picture, and/or any kindof a picture generating device, for example a computer-graphicsprocessor for generating a computer animated picture, or any kind ofdevice for obtaining and/or providing a real-world picture, a computeranimated picture (e.g., a screen content, a virtual reality (VR)picture) and/or any combination thereof (e.g., an augmented reality (AR)picture). In the following, all these kinds of pictures and any otherkind of picture will be referred to as “picture”, unless specificallydescribed otherwise, while the previous explanations with regard to theterm “picture” covering “video pictures”, “video images”, “stillimages”, and “still pictures” still hold true, unless explicitlyspecified differently.

A (digital) picture is or can be regarded as a two-dimensional array ormatrix of samples with intensity values. A sample in the array may alsobe referred to as pixel (short form of picture element) or a pel. Thenumber of samples in horizontal and vertical direction (or axis) of thearray or picture define the size and/or resolution of the picture. Forrepresentation of color, typically three color components are employed,i.e. the picture may be represented or include three sample arrays. InRBG format or color space a picture comprises a corresponding red, greenand blue sample array. However, in video coding each pixel is typicallyrepresented in a luminance/chrominance format or color space, e.g.,YCbCr, which comprises a luminance component indicated by Y (sometimesalso L is used instead) and two chrominance components indicated by Cband Cr. The luminance (or short luma) component Y represents thebrightness or grey level intensity (e.g., like in a grey-scale picture),while the two chrominance (or short chroma) components Cb and Crrepresent the chromaticity or color information components. Accordingly,a picture in YCbCr format comprises a luminance sample array ofluminance sample values (Y), and two chrominance sample arrays ofchrominance values (Cb and Cr). Pictures in RGB format may be convertedor transformed into YCbCr format and vice versa, the process is alsoknown as color transformation or conversion. If a picture is monochrome,the picture may comprise only a luminance sample array.

The picture source 312 may be, for example a camera for capturing apicture, a memory, e.g., a picture memory, comprising or storing apreviously captured or generated picture, and/or any kind of interface(internal or external) to obtain or receive a picture. The camera maybe, for example, a local or integrated camera integrated in the sourcedevice, the memory may be a local or integrated memory, e.g., integratedin the source device. The interface may be, for example, an externalinterface to receive a picture from an external video source, forexample an external picture capturing device like a camera, an externalmemory, or an external picture generating device, for example anexternal computer-graphics processor, computer or server. The interfacecan be any kind of interface, e.g., a wired or wireless interface, anoptical interface, according to any proprietary or standardizedinterface protocol. The interface for obtaining the picture data 312 maybe the same interface as or a part of the communication interface 318.

In distinction to the pre-processing unit 314 and the processingperformed by the pre-processing unit 314, the picture or picture data313 may also be referred to as raw picture or raw picture data 313.

Pre-processing unit 314 is configured to receive the (raw) picture data313 and to perform pre-processing on the picture data 313 to obtain apre-processed picture 315 or pre-processed picture data 315.Pre-processing performed by the pre-processing unit 314 may, e.g.,comprise trimming, color format conversion (e.g., from RGB to YCbCr),color correction, or de-noising.

The encoder 100 is configured to receive the pre-processed picture data315 and provide encoded picture data 171 (further details will bedescribed, e.g., based on FIG. 1).

Communication interface 318 of the source device 310 may be configuredto receive the encoded picture data 171 and to directly transmit it toanother device, e.g., the destination device 320 or any other device,for storage or direct reconstruction, or to process the encoded picturedata 171 for respectively before storing the encoded data 330 and/ortransmitting the encoded data 330 to another device, e.g., thedestination device 320 or any other device for decoding or storing.

The destination device 320 comprises a decoder 200 or decoding unit 200,and may additionally, i.e. optionally, comprise a communicationinterface or communication unit 322, a post-processing unit 326 and adisplay device 328.

The communication interface 322 of the destination device 320 isconfigured to receive the encoded picture data 171 or the encoded data330, e.g., directly from the source device 310 or from any other source,e.g., a memory, e.g., an encoded picture data memory.

The communication interface 318 and the communication interface 322 maybe configured to transmit respectively receive the encoded picture data171 or encoded data 330 via a direct communication link between thesource device 310 and the destination device 320, e.g., a direct wiredor wireless connection, or via any kind of network, e.g., a wired orwireless network or any combination thereof, or any kind of private andpublic network, or any kind of combination thereof.

The communication interface 318 may be, e.g., configured to package theencoded picture data 171 into an appropriate format, e.g., packets, fortransmission over a communication link or communication network, and mayfurther comprise data loss protection and data loss recovery.

The communication interface 322, forming the counterpart of thecommunication interface 318, may be, e.g., configured to de-package theencoded data 330 to obtain the encoded picture data 171 and may furtherbe configured to perform data loss protection and data loss recovery,e.g., comprising error concealment.

Both, communication interface 318 and communication interface 322 may beconfigured as unidirectional communication interfaces as indicated bythe arrow for the encoded picture data 330 in FIG. 3 pointing from thesource device 310 to the destination device 320, or bi-directionalcommunication interfaces, and may be configured, e.g., to send andreceive messages, e.g., to set up a connection, to acknowledge and/orre-send lost or delayed data including picture data, and exchange anyother information related to the communication link and/or datatransmission, e.g., encoded picture data transmission.

The decoder 200 is configured to receive the encoded picture data 171and provide decoded picture data 231 or a decoded picture 231 (furtherdetails will be described, e.g., based on FIG. 2).

The post-processor 326 of destination device 320 is configured topost-process the decoded picture data 231, e.g., the decoded picture231, to obtain post-processed picture data 327, e.g., a post-processedpicture 327. The post-processing performed by the post-processing unit326 may comprise, e.g., color format conversion (e.g., from YCbCr toRGB), color correction, trimming, or re-sampling, or any otherprocessing, e.g., for preparing the decoded picture data 231 fordisplay, e.g., by display device 328.

The display device 328 of the destination device 320 is configured toreceive the post-processed picture data 327 for displaying the picture,e.g., to a user or viewer. The display device 328 may be or comprise anykind of display for representing the reconstructed picture, e.g., anintegrated or external display or monitor. The displays may, e.g.,comprise cathode ray tubes (CRT), liquid crystal displays (LCD), plasmadisplays, organic light emitting diodes (OLED) displays or any kind ofother display, beamer, or hologram (3D).

Although FIG. 3 depicts the source device 310 and the destination device320 as separate devices, embodiments of devices may also comprise bothor both functionalities, the source device 310 or correspondingfunctionality and the destination device 320 or correspondingfunctionality. In such embodiments the source device 310 orcorresponding functionality and the destination device 320 orcorresponding functionality may be implemented using the same hardwareand/or software or by separate hardware and/or software or anycombination thereof.

As will be apparent for the skilled person based on the description, theexistence and (exact) split of functionalities of the different units orfunctionalities within the source device 310 and/or destination device320 as shown in FIG. 3 may vary depending on the actual device andapplication.

Therefore, the source device 310 and the destination device 320 as shownin FIG. 3 are just example embodiments of the invention and embodimentsof the invention are not limited to those shown in FIG. 3.

Source device 310 and destination device 320 may comprise any of a widerange of devices, including any kind of handheld or stationary devices,e.g., notebook or laptop computers, mobile phones, smart phones, tabletsor tablet computers, cameras, desktop computers, set-top boxes,televisions, display devices, digital media players, video gamingconsoles, video streaming devices, broadcast receiver device, or thelike, and may use no or any kind of operating system.

Encoder & Encoding Method

FIG. 1 shows a schematic/conceptual block diagram of an embodiment of anencoder 100, e.g., a picture encoder 100, which comprises an input 102,a residual calculation unit 104, a transformation unit 106, aquantization unit 108, an inverse quantization unit 110, and inversetransformation unit 112, a reconstruction unit 114, a buffer 118, a loopfilter 120, a decoded picture buffer (DPB) 130, a prediction unit 160(including an inter estimation unit 142, an inter-prediction unit 144,an intra-estimation unit 152, and an intra-prediction unit 154) a modeselection unit 162, an entropy encoding unit 170, and an output 172. Avideo encoder 100 as shown in FIG. 1 may also be referred to as hybridvideo encoder or a video encoder according to a hybrid video codec.

For example, the residual calculation unit 104, the transformation unit106, the quantization unit 108, and the entropy encoding unit 170 form aforward signal path of the encoder 100, whereas, for example, theinverse quantization unit 110, the inverse transformation unit 112, thereconstruction unit 114, the buffer 118, the loop filter 120, thedecoded picture buffer (DPB) 130, the inter-prediction unit 144, and theintra-prediction unit 154 form a backward signal path of the encoder,wherein the backward signal path of the encoder corresponds to thesignal path of the decoder (see decoder 200 in FIG. 2).

The encoder 100 is configured to receive, e.g., by input 102, a picture101 or a picture block 103 of the picture 101, e.g., picture of asequence of pictures forming a video or video sequence. The pictureblock 103 may also be referred to as current picture block or pictureblock to be coded, and the picture 101 as current picture or picture tobe coded (in particular in video coding to distinguish the currentpicture from other pictures, e.g., previously encoded and/or decodedpictures of the same video sequence, i.e. the video sequence which alsocomprises the current picture).

Residual Calculation

The residual calculation unit 104 is configured to calculate a residualblock 105 based on the picture block 103 and a prediction block 165(further details about the prediction block 165 are provided later),e.g., by subtracting sample values of the prediction block 165 fromsample values of the picture block 103, sample by sample (pixel bypixel) to obtain the residual block 105 in the sample domain.

Transformation

The transformation unit 106 is configured to apply a transformation,e.g., a spatial frequency transform or a linear spatial (frequency)transform, e.g., a discrete cosine transform (DCT) or discrete sinetransform (DST), on the sample values of the residual block 105 toobtain transformed coefficients 107 in a transform domain. Thetransformed coefficients 107 may also be referred to as transformedresidual coefficients and represent the residual block 105 in thetransform domain.

The transformation unit 106 may be configured to apply integerapproximations of DCT/DST, such as the core transforms specified forHEVC/H.265. Compared to an orthonormal DCT transform, such integerapproximations are typically scaled by a certain factor. In order topreserve the norm of the residual block which is processed by forwardand inverse transforms, additional scaling factors are applied as partof the transform process. The scaling factors are typically chosen basedon certain constraints like scaling factors being a power of two forshift operation, bit depth of the transformed coefficients, trade-offbetween accuracy and implementation costs, etc. Specific scaling factorsare, for example, specified for the inverse transform, e.g., by inversetransformation unit 212, at a decoder 200 (and the corresponding inversetransform, e.g., by inverse transformation unit 112 at an encoder 100)and corresponding scaling factors for the forward transform, e.g., bytransformation unit 106, at an encoder 100 may be specified accordingly.

Quantization

The quantization unit 108 is configured to quantize the transformedcoefficients 107 to obtain quantized coefficients 109, e.g., by applyingscalar quantization or vector quantization. The quantized coefficients109 may also be referred to as quantized residual coefficients 109. Forexample for scalar quantization, different scaling may be applied toachieve finer or coarser quantization. Smaller quantization step sizescorrespond to finer quantization, whereas larger quantization step sizescorrespond to coarser quantization. The applicable quantization stepsize may be indicated by a quantization parameter (QP). The quantizationparameter may for example be an index to a predefined set of applicablequantization step sizes. For example, small quantization parameters maycorrespond to fine quantization (small quantization step sizes) andlarge quantization parameters may correspond to coarse quantization(large quantization step sizes) or vice versa. The quantization mayinclude division by a quantization step size and corresponding inversedequantization, e.g., by inverse quantization 110, may includemultiplication by the quantization step size. Embodiments according toHEVC, may be configured to use a quantization parameter to determine thequantization step size. Generally, the quantization step size may becalculated based on a quantization parameter using a fixed pointapproximation of an equation including division. Additional scalingfactors may be introduced for quantization and dequantization to restorethe norm of the residual block, which might get modified because of thescaling used in the fixed point approximation of the equation forquantization step size and quantization parameter. In one exampleimplementation, the scaling of the inverse transform and dequantizationmight be combined. Alternatively, customized quantization tables may beused and signaled from an encoder to a decoder, e.g., in a bitstream.The quantization is a lossy operation, wherein the loss increases withincreasing quantization step sizes.

Embodiments of the encoder 100 (or respectively of the quantization unit108) may be configured to output the quantization scheme andquantization step size, e.g., by means of the corresponding quantizationparameter, so that a decoder 200 may receive and apply the correspondinginverse quantization. Embodiments of the encoder 100 (or quantizationunit 108) may be configured to output the quantization scheme andquantization step size, e.g., directly or entropy encoded via theentropy encoding unit 170 or any other entropy coding unit.

The inverse quantization unit 110 is configured to apply the inversequantization of the quantization unit 108 on the quantized coefficientsto obtain dequantized coefficients 11, e.g., by applying the inverse ofthe quantization scheme applied by the quantization unit 108 based on orusing the same quantization step size as the quantization unit 108. Thedequantized coefficients 11 may also be referred to as dequantizedresidual coefficients 11 and correspond—although typically not identicalto the transformed coefficients due to the loss by quantization—to thetransformed coefficients 108.

The inverse transformation unit 112 is configured to apply the inversetransformation of the transformation applied by the transformation unit106, e.g., an inverse discrete cosine transform (DCT) or inversediscrete sine transform (DST), to obtain an inverse transformed block113 in the sample domain. The inverse transformed block 113 may also bereferred to as inverse transformed dequantized block 113 or inversetransformed residual block 113.

The reconstruction unit 114 is configured to combine the inversetransformed block 113 and the prediction block 165 to obtain areconstructed block 115 in the sample domain, e.g., by sample-wiseadding the sample values of the decoded residual block 113 and thesample values of the prediction block 165.

The buffer unit 116 (or short “buffer” 116), e.g., a line buffer 116, isconfigured to buffer or store the reconstructed block and the respectivesample values, for example for intra-estimation and/or intra-prediction.In further embodiments, the encoder may be configured to use unfilteredreconstructed blocks and/or the respective sample values stored inbuffer unit 116 for any kind of estimation and/or prediction.

The loop filter unit 120 (or short “loop filter” 120), is configured tofilter the reconstructed block 115 to obtain a filtered block 121, e.g.,by applying a de-blocking sample-adaptive offset (SAO) filter or otherfilters, e.g., sharpening or smoothing filters or collaborative filters.The filtered block 121 may also be referred to as filtered reconstructedblock 121.

Embodiments of the loop filter unit 120 may comprise (not shown inFIG. 1) a filter analysis unit and the actual filter unit, wherein thefilter analysis unit is configured to determine loop filter parametersfor the actual filter. The filter analysis unit may be configured toapply fixed pre-determined filter parameters to the actual loop filter,adaptively select filter parameters from a set of predetermined filterparameters or adaptively calculate filter parameters for the actual loopfilter.

Embodiments of the loop filter unit 120 may comprise (not shown inFIG. 1) one or a plurality of filters (loop filtercomponents/subfilters), e.g., one or more of different kinds or types offilters, e.g., connected in series or in parallel or in any combinationthereof, wherein each of the filters may comprise individually orjointly with other filters of the plurality of filters a filter analysisunit to determine the respective loop filter parameters, e.g., asdescribed in the previous paragraph.

Embodiments of the encoder 100 (respectively loop filter unit 120) maybe configured to output the loop filter parameters, e.g., directly orentropy encoded via the entropy encoding unit 170 or any other entropycoding unit, so that, e.g., a decoder 200 may receive and apply the sameloop filter parameters for decoding.

The decoded picture buffer (DPB) 130 is configured to receive and storethe filtered block 121. The decoded picture buffer 130 may be furtherconfigured to store other previously filtered blocks, e.g., previouslyreconstructed and filtered blocks 121, of the same current picture or ofdifferent pictures, e.g., previously reconstructed pictures, and mayprovide complete previously reconstructed, i.e. decoded, pictures (andcorresponding reference blocks and samples) and/or a partiallyreconstructed current picture (and corresponding reference blocks andsamples), for example for inter-estimation and/or inter-prediction.

Further embodiments of the invention may also be configured to use thepreviously filtered blocks and corresponding filtered sample values ofthe decoded picture buffer 130 for any kind of estimation or prediction,e.g., intra- and inter-estimation and prediction.

Motion Estimation and Prediction

The prediction unit 160, also referred to as block prediction unit 160,is configured to receive or obtain the picture block 103 (currentpicture block 103 of the current picture 101) and decoded or at leastreconstructed picture data, e.g., reference samples of the same(current) picture from buffer 116 and/or decoded picture data 231 fromone or a plurality of previously decoded pictures from decoded picturebuffer 130, and to process such data for prediction, i.e. to provide aprediction block 165, which may be an inter-predicted block 145 or anintra-predicted block 155.

Mode selection unit 162 may be configured to select a prediction mode(e.g., an intra- or inter-prediction mode) and/or a correspondingprediction block 145 or 155 to be used as prediction block 165 for thecalculation of the residual block 105 and for the reconstruction of thereconstructed block 115.

Embodiments of the mode selection unit 162 may be configured to selectthe prediction mode (e.g., from those supported by prediction unit 160),which provides the best match or in other words the minimum residual(minimum residual means better compression for transmission or storage),or a minimum signaling overhead (minimum signaling overhead means bettercompression for transmission or storage), or which considers or balancesboth. The mode selection unit 162 may be configured to determine theprediction mode based on rate distortion optimization (RDO), i.e. selectthe prediction mode which provides a minimum rate distortionoptimization or which associated rate distortion at least a fulfills aprediction mode selection criterion.

In the following the prediction processing (e.g., prediction unit 160)and mode selection (e.g., by mode selection unit 162) performed by anexample encoder 100 will be explained in more detail.

As described above, encoder 100 is configured to determine or select thebest or an optimum prediction mode from a set of (pre-determined)prediction modes. The set of prediction modes may comprise, e.g.,intra-prediction modes and/or inter-prediction modes.

The set of intra-prediction modes may comprise 32 differentintra-prediction modes, e.g., non-directional modes like DC (or mean)mode and planar mode, or directional modes, e.g., as defined in H.264,or may comprise 65 different intra-prediction modes, e.g.,non-directional modes like DC (or mean) mode and planar mode, ordirectional modes, e.g., as defined in H.265.

The set of (or possible) inter-prediction modes depend on the availablereference pictures (i.e. previous at least partially decoded pictures,e.g., stored in DBP 230) and other inter-prediction parameters, e.g.,whether the whole reference picture or only a part, e.g., a searchwindow area around the area of the current block, of the referencepicture is used for searching for a best matching reference block,and/or e.g., whether pixel interpolation is applied, e.g., half/semi-peland/or quarter-pel interpolation, or not.

In addition to the above prediction modes, skip mode and/or direct modemay be applied.

The prediction unit 160 may be further configured to partition the block103 into smaller block partitions or sub-blocks, e.g., iteratively usingquad-tree-partitioning (QT), binary partitioning (BT) ortriple-tree-partitioning (TT) or any combination thereof, and toperform, e.g., the prediction for each of the block partitions orsub-blocks, wherein the mode selection comprises the selection of thetree-structure of the partitioned block 103 and the prediction modesapplied to each of the block partitions or sub-blocks.

The inter-estimation unit 142, also referred to as inter pictureestimation unit 142, is configured to receive or obtain the pictureblock 103 (current picture block 103 of the current picture 101) and adecoded picture 231, or at least one or a plurality of previouslyreconstructed blocks, e.g., reconstructed blocks of one or a pluralityof other/different previously decoded pictures 231, for inter-estimation(or “inter picture estimation”). E.g., a video sequence may comprise thecurrent picture and the previously decoded pictures 231, or in otherwords, the current picture and the previously decoded pictures 231 maybe part of or form a sequence of pictures forming a video sequence.

The encoder 100 may, e.g., be configured to select a reference blockfrom a plurality of reference blocks of the same or different picturesof the plurality of other pictures and provide a reference picture (orreference picture index) and/or an offset (spatial offset) between theposition (x, y coordinates) of the reference block and the position ofthe current block as inter-estimation parameters 143 to theinter-prediction unit 144. This offset is also called motion vector(MV). The inter-estimation is also referred to as motion estimation (ME)and the inter-prediction also motion prediction (MP).

The inter-prediction unit 144 is configured to obtain, e.g., receive, aninter-prediction parameter 143 and to perform inter-prediction based onor using the inter-prediction parameter 143 to obtain aninter-prediction block 145.

Although FIG. 1 shows two distinct units (or steps) for theinter-coding, namely inter estimation 142 and inter-prediction 152, bothfunctionalities may be performed as one (inter estimation typicallycomprises calculating an/the inter-prediction block, i.e. the or a “kindof” inter-prediction 152), e.g., by testing all possible or apredetermined subset of possible interprediction modes iteratively whilestoring the currently best inter-prediction mode and respectiveinter-prediction block, and using the currently best inter-predictionmode and respective inter-prediction block as the (final)inter-prediction parameter 143 and inter-prediction block 145 withoutperforming another time the inter-prediction 144.

The intra-estimation unit 152 is configured to obtain, e.g., receive,the picture block 103 (current picture block) and one or a plurality ofpreviously reconstructed blocks, e.g., reconstructed neighbor blocks, ofthe same picture for intra-estimation. The encoder 100 may, e.g., beconfigured to select an intra-prediction mode from a plurality ofintra-prediction modes and provide it as intra-estimation parameter 153to the intra-prediction unit 154.

Embodiments of the encoder 100 may be configured to select theintra-prediction mode based on an optimization criterion, e.g., minimumresidual (e.g., the intra-prediction mode providing the prediction block155 most similar to the current picture block 103) or minimum ratedistortion.

The intra-prediction unit 154 is configured to determine based on theintra-prediction parameter 153, e.g., the selected intra-prediction mode153, the intra-prediction block 155.

Although FIG. 1 shows two distinct units (or steps) for theintra-coding, namely intra-estimation 152 and intra-prediction 154, bothfunctionalities may be performed as one (intra-estimation typicallycomprises calculating the intra-prediction block, i.e. the or a “kindof” intra-prediction 154), e.g., by testing all possible or apredetermined subset of possible intra-prediction modes iterativelywhile storing the currently best intra-prediction mode and respectiveintra-prediction block, and using the currently best intra-predictionmode and respective intra-prediction block as the (final)intra-prediction parameter 153 and intra-prediction block 155 withoutperforming another time the intra-prediction 154.

The invention, as explained further below with respect to the device 500(FIG. 5) and method 700 (FIG. 7) according to embodiments of theinvention may be applied at this position of the encoder 100. That is,the device 500 may be or be part of the encoder 100, specifically theintra-prediction unit 154.

The entropy encoding unit 170 is configured to apply an entropy encodingalgorithm or scheme (e.g., a variable length coding (VLC) scheme, ancontext adaptive VLC scheme (CALVC), an arithmetic coding scheme, acontext adaptive binary arithmetic coding (CABAC) on the quantizedresidual coefficients 109, inter-prediction parameters 143,intra-prediction parameter 153, and/or loop filter parameters,individually or jointly (or not at all) to obtain encoded picture data171 which can be output by the output 172, e.g., in the form of anencoded bitstream 171.

FIG. 2 shows an exemplary video decoder 200 configured to receiveencoded picture data (e.g., encoded bitstream) 171, e.g., encoded byencoder 100, to obtain a decoded picture 231.

The decoder 200 comprises an input 202, an entropy decoding unit 204, aninverse quantization unit 210, an inverse transformation unit 212, areconstruction unit 214, a buffer 216, a loop filter 220, a decodedpicture buffer 230, a prediction unit 260 (including an inter-predictionunit 244, and an intra-prediction unit 254), a mode selection unit 260and an output 232.

The entropy decoding unit 204 is configured to perform entropy decodingto the encoded picture data 171 to obtain, e.g., quantized coefficients209 and/or decoded coding parameters (not shown in FIG. 2), e.g.,(decoded) any or all of inter-prediction parameters 143,intra-prediction parameter 153, and/or loop filter parameters.

In embodiments of the decoder 200, the inverse quantization unit 210,the inverse transformation unit 212, the reconstruction unit 214, thebuffer 216, the loop filter 220, the decoded picture buffer 230, theprediction unit 260 and the mode selection unit 260 are configured toperform the inverse processing of the encoder 100 (and the respectivefunctional units) to decode the encoded picture data 171.

In particular, the inverse quantization unit 210 may be identical infunction to the inverse quantization unit 110, the inversetransformation unit 212 may be identical in function to the inversetransformation unit 112, the reconstruction unit 214 may be identical infunction reconstruction unit 114, the buffer 216 may be identical infunction to the buffer 116, the loop filter 220 may be identical infunction to the loop filter 120 (with regard to the actual loop filteras the loop filter 220 typically does not comprise a filter analysisunit to determine the filter parameters based on the original image 101or block 103 but receives (explicitly or implicitly) or obtains thefilter parameters used for encoding, e.g., from entropy decoding unit204), and the decoded picture buffer 230 may be identical in function tothe decoded picture buffer 130.

The prediction unit 260 may comprise an inter-prediction unit 244 and anintra-prediction unit 254, wherein the inter-prediction unit 144 may beidentical in function to the inter-prediction unit 244, and theintra-prediction unit 154 may be identical in function to theintra-prediction unit 254. The prediction unit 260 and the modeselection unit 262 are typically configured to perform the blockprediction and/or obtain the predicted block 265 from the encoded data171 only (without any further information about the original image 101)and to receive or obtain (explicitly or implicitly) the predictionparameters 143 or 153 and/or the information about the selectedprediction mode, e.g., from the entropy decoding unit 204.

The invention, as explained further below with respect to the device 500(see FIG. 5) and method 700 (see FIG. 7) according to embodiments of theinvention may be applied at this position of the decoder 200. That is,the device 500 may be or be part of the decoder 200, specifically theintra-prediction unit 154.

The decoder 200 is configured to output the decoded picture 230, e.g.,via output 232, for presentation or viewing to a user.

With reference to FIGS. 14 and 15, FIG. 4 illustrates more specificallyin (a) the source of discontinuities that can be removed by theembodiments of the invention. In particular, the reason for thesediscontinuities is that two vertically adjacent prediction samples 401in a prediction block 400 (e.g., PU or TU) may be predicted fromreference samples 403 that are not adjacent to each other due to anacute intra-prediction angle, which is an interpolation flaw. While thisflaw may partially be reduced by applying a reference samples smoothingfilter or an intra-interpolation filter with a length N_(f), a fixedlength may not be large enough in the case of an intra-prediction angleof significantly less than 45°. The filtering process can reducesdiscontinuity effects by convoluting the reference samples 403 shown inFIG. 4 during the filtering process. However, discontinuities may stilloccur, if the reference samples 403 selected for the vertically adjacentprediction samples 401 are too far apart. An example of suchdiscontinuities, which can be visually observed e.g., for the case ofsynthesized reference (the upper row), is shown in (b).

FIG. 5 shows schematically a device 500 according to an embodiment ofthe invention, which is configured to intra-predict a prediction block400 of a video image in an improved manner. Namely, the device 500 doesthereby not suffer from the above-described source of thediscontinuities shown in FIG. 4. The device 500 may be or be part of theencoder 100 or decoder 200 shown in FIG. 1 or FIG. 2, respectively,specifically the intra-prediction units 154 or 254.

The device 500 is configured to perform several functions, which may,for instance, be implemented by means of a processor or other kind ofprocessing circuitry. Specifically, the device 500 is configured toselect a directional intra-prediction mode 501 a from a set ofdirectional intra-prediction modes 501, wherein each directionalintra-prediction mode 501 corresponds to a different intra-predictionangle. These directional intra-prediction modes 501 may include thedirectional/angular intra-prediction modes shown in FIG. 8 (and asdefined in the standard). The intra-prediction angle bases on thedirection of intra-predicting a prediction sample 401 from a referencesample 403. For instance, the angle may be defined between thisintra-prediction direction and an upper edge (horizontal edge) of theprediction block 400.

The device 500 is further configured to determine, for a givenprediction sample 401 of the prediction block 400, a reference sample403 a from a set of reference samples 403 based on the selecteddirectional intra-prediction mode 501 a, and apply a filter 402 to thedetermined reference sample 403 a. The filter 402 may be a filter with acertain pre-selected filter length. The filter length may, for instance,be pre-selected based on the aspect ratio, width, and/or height of theprediction block 400. The device 500 may be configured to proceed in theabove-described way for each prediction sample 401 of the predictionblock 400. That is, for each prediction sample 401, the device 500 maydetermine a reference sample 403 a from the reference samples 403, andmay apply the filter 402 to each determined reference sample 403.Thereby, the device 500 is able to intra-predict the entire predictionblock 400.

In the device 500, within at least a subset 600 (see better FIG. 6) ofthe directional intra-prediction modes 501, an angular step 601 (seebetter FIG. 6) between the directional intra-prediction modes 501 isdefined in dependence of a filter length of the filter 402. The subset600 may include “extended” directional intra-prediction modes 501, i.e.directional intra-prediction modes added to those shown in FIG. 8, e.g.,as illustrated in FIG. 13. In particular, for a rectangular predictionblock 400, the directional intra-prediction modes 501 in the subset 600may include modes 501 that relate to acute intra-prediction angles(angles smaller than 45°). The number of directional intra-predictionmodes 501 in the subset 600 may be selected based on an aspect ratio,height, and/or width of the prediction block 400, and is assumed to begiven for a certain prediction block 400. Each directionalintra-prediction mode 501 in the subset 600 may be provided with anindex to be signaled (like the indices of the other modes 501).

FIG. 6 shows exemplarily multiple directional intra-prediction modes 501in the set, and specifically shows directional intra-prediction modes inthe subset 600 contained in the set. The subset 600 of directionalintra-prediction modes 501 is defined by ΔM_(o), wherein ΔM₀ denotes thenumber of directional intra-prediction modes 501 in the subset 600. Theintra-prediction angles associated with the modes 501 in the subset 600span an angular range that depends on the angular step 601 (betweenangles of neighboring directional intra-prediction modes 501). Theangular step 601 is defined in accordance with the reference samplefilter 402 length. The idea here is to prevent the occurrence ofsituations shown in FIG. 4 by decreasing the angular step 601 within thesubset 600, and thus the overall angular range spanned by the subset600, for smaller filter length. The angular step 601 between anglesassociated with directional intra-prediction modes 501 in the subset 600may notably differ, particularly may be larger or smaller, than anangular step between angles associated with directional intra-predictionmodes 501 not in the subset 600 (but still in the set of modes 501).

Specifically, the angular step 601 in the subset 600 may be specified asfollows:

${\Delta\alpha} = \frac{\arctan N_{f}}{\Delta M_{0}}$

wherein Δα is the angular step 601, N_(f) denotes the length of thefilter 402 (implying intra-interpolation filter). The subset 600, i.e.the number of modes 501 in the subset 600, may specifically be definedaccording to e.g., an aspect ratio

$R_{A_{0}} = \frac{W_{0}}{H_{0}}$of the prediction block 400. Also the filter length of the filter 402may be selected in dependence of the aspect ratio of the predictionblock 400 or may be selected in dependence of a height and/or a width ofthe prediction block 400. Notably, the last directional intra-predictionmode 501 in the subset 600 will be associated with an intra-predictionangle calculated by Δα·ΔM₀, and does not necessary fall to the top-rightcorner (as it is shown in FIG. 6).

Below table shows exemplary angular steps 601 that may be appliedbetween the angles associated with the directional intra-predictionmodes 501 in the subset 600. As can be seen in the table, the angularstep 601 may be between 2-4 degrees, particularly it may be 2, 2.5, 3 or4 degrees, depending on the filter length of the filter 402. The filterlength of the filter 402 may span 1-7 adjacent reference samples 403,particularly it may span 1, 3, 5 or 7 adjacent reference samples 403.

Length of reference sample filter 1 3 5 7 Angular step (degrees) 2 2.5 34

It can be seen that the more taps the filters 402 have (i.e. largerfilter length in terms of reference samples 403), the larger the angularstep 601 that is used for the directional intra-prediction modes 501 inthe subset 600. A main difference to current implementations andproposals is notably that the intra-prediction directions of thedirectional intra-prediction modes 501 in the subset 600 do notnecessarily correspond to opposite intra-prediction directions of othermodes 501 in the set, but depend only the selected filter 402, i.e. itsfilter length.

FIG. 7 shows a method 700 according to an embodiment of the invention.The method 700 is for intra-predicting a prediction block 400 of a videoimage, and may be carried out by the device 500 shown in FIG. 5. Inparticular, the method 700 comprises a step 701 of selecting adirectional intra-prediction mode 501 a from a set of directionalintra-prediction modes 501, wherein each directional intra-predictionmode 501 corresponds to a different intra-prediction angle. Further, themethod 700 comprises a step 701 of determining, for a given predictionsample 401 of the prediction block 400, a reference sample 403 a from aset of reference samples 403 based on the selected directionalintra-prediction mode 501 a, and applying a filter 402 to the determinedreference sample 403 a. In the method 700, within a subset 600 of thedirectional intra-prediction modes 501, an angular step 601 between thedirectional intra-prediction modes 501 is defined in dependence of afilter length of the filter 402.

Note that this specification provides explanations for pictures(frames), but fields substitute as pictures in the case of an interlacepicture signal.

Although embodiments of the invention have been primarily describedbased on video coding, it should be noted that embodiments of theencoder 100 and decoder 200 (and correspondingly the system 300) mayalso be configured for still picture processing or coding, i.e. theprocessing or coding of an individual picture independent of anypreceding or consecutive picture as in video coding. In general onlyinter-estimation 142, inter-prediction 144, 242 are not available incase the picture processing coding is limited to a single picture 101.Most if not all other functionalities (also referred to as tools ortechnologies) of the video encoder 100 and video decoder 200 may equallybe used for still pictures, e.g., partitioning, transformation (scaling)106, quantization 108, inverse quantization 110, inverse transformation112, intra-estimation 142, intra-prediction 154, 254 and/or loopfiltering 120, 220, and entropy coding 170 and entropy decoding 204.

The person skilled in the art will understand that the “blocks”(“units”) of the various figures (method and apparatus) represent ordescribe functionalities of embodiments of the invention (rather thannecessarily individual “units” in hardware or software) and thusdescribe equally functions or features of apparatus embodiments as wellas method embodiments (unit=step).

The terminology of “units” is merely used for illustrative purposes ofthe functionality of embodiments of the encoder/decoder and are notintended to limiting the disclosure.

In the several embodiments provided in the present application, itshould be understood that the disclosed system, apparatus, and methodmay be implemented in other manners. For example, the describedapparatus embodiment is merely exemplary. For example, the unit divisionis merely logical function division and may be other division in actualimplementation. For example, a plurality of units or components may becombined or integrated into another system, or some features may beignored or not performed. In addition, the displayed or discussed mutualcouplings or direct couplings or communication connections may beimplemented by using some interfaces. The indirect couplings orcommunication connections between the apparatuses or units may beimplemented in electronic, mechanical, or other forms.

The units described as separate parts may or may not be physicallyseparate, and parts displayed as units may or may not be physical units,may be located in one position, or may be distributed on a plurality ofnetwork units. Some or all of the units may be selected according toactual needs to achieve the objectives of the solutions of theembodiments.

In addition, functional units in the embodiments of the presentinvention may be integrated into one processing unit, or each of theunits may exist alone physically, or two or more units are integratedinto one unit.

Embodiments of the invention may further comprise an apparatus, e.g.,encoder and/or decoder, which comprises a processing circuitryconfigured to perform any of the methods and/or processes describedherein.

Embodiments of the encoder 100 and/or decoder 200 may be implemented ashardware, firmware, software or any combination thereof. For example,the functionality of the encoder/encoding or decoder/decoding may beperformed by a processing circuitry with or without firmware orsoftware, e.g., a processor, a microcontroller, a digital signalprocessor (DSP), a field programmable gate array (FPGA), anapplication-specific integrated circuit (ASIC), or the like.

The functionality of the encoder 100 (and corresponding encoding method100) and/or decoder 200 (and corresponding decoding method 200) may beimplemented by program instructions stored on a computer readablemedium. The program instructions, when executed, cause a processingcircuitry, computer, processor or the like, to perform the steps of theencoding and/or decoding methods. The computer readable medium can beany medium, including non-transitory storage media, on which the programis stored such as a Blu ray disc, DVD, CD, USB (flash) drive, hard disc,server storage available via a network, etc.

An embodiment of the invention comprises or is a computer programcomprising program code for performing any of the methods describedherein, when executed on a computer.

An embodiment of the invention comprises or is a computer readablemedium comprising a program code that, when executed by a processor,causes a computer system to perform any of the methods described herein.

LIST OF REFERENCE SIGNS

-   FIG. 1-   100 Encoder-   103 Picture block-   102 Input (e.g., input port, input interface)-   104 Residual calculation [unit or step]-   105 Residual block-   106 Transformation (e.g., additionally comprising scaling) [unit or    step]-   107 Transformed coefficients-   108 Quantization [unit or step]-   109 Quantized coefficients-   110 Inverse quantization [unit or step]-   111 De-quantized coefficients-   112 Inverse transformation (e.g., additionally comprising scaling)    [unit or step]-   113 Inverse transformed block-   114 Reconstruction [unit or step]-   115 Reconstructed block-   116 (Line) buffer [unit or step]-   117 Reference samples-   120 Loop filter [unit or step]-   121 Filtered block-   130 Decoded picture buffer (DPB) [unit or step]-   142 Inter estimation (or inter picture estimation) [unit or step]-   143 Inter estimation parameters (e.g., reference picture/reference    picture index, motion vector/offset)-   144 Inter prediction (or inter picture prediction) [unit or step]-   145 Inter prediction block-   152 Intra estimation (or intra picture estimation) [unit or step]-   153 Intra prediction parameters (e.g., intra prediction mode)-   154 Intra prediction (intra frame/picture prediction) [unit or step]-   155 Intra prediction block-   162 Mode selection [unit or step]-   165 Prediction block (either inter prediction block 145 or intra    prediction block 155)-   170 Entropy encoding [unit or step]-   171 Encoded picture data (e.g., bitstream)-   172 Output (output port, output interface)-   231 Decoded picture-   FIG. 2-   200 Decoder-   171 Encoded picture data (e.g., bitstream)-   202 Input (port/interface)-   204 Entropy decoding-   209 Quantized coefficients-   210 Inverse quantization-   211 De-quantized coefficients-   212 Inverse transformation (scaling)-   213 Inverse transformed block-   214 Reconstruction (unit)-   215 Reconstructed block-   216 (Line) buffer-   217 Reference samples-   220 Loop filter (in loop filter)-   221 Filtered block-   230 Decoded picture buffer (DPB)-   231 Decoded picture-   232 Output (port/interface)-   244 Inter prediction (inter frame/picture prediction)-   245 Inter prediction block-   254 Intra prediction (intra frame/picture prediction)-   255 Intra prediction block-   260 Mode selection-   265 Prediction block (inter prediction block 245 or intra prediction    block 255)-   FIG. 3-   300 Coding system-   310 Source device-   312 Picture Source-   313 (Raw) picture data-   314 Pre-processor/Pre-processing unit-   315 Pre-processed picture data-   318 Communication unit/interface-   320 Destination device-   322 Communication unit/interface-   326 Post-processor/Post-processing unit-   327 Post-processed picture data-   328 Display device/unit-   330 transmitted/received/communicated (encoded) picture data-   FIG. 4-   400 Prediction block-   401 Prediction sample-   402 Filter-   403 Reference Sample-   FIG. 5-   402 Filter-   403 Reference sample-   403 a Determined reference sample-   500 Device-   501 Directional intra-prediction modes-   501 a Selected directional intra-prediction mode-   600 Subset of directional intra-prediction modes-   FIG. 6-   400 Prediction block-   501 Directional intra-prediction modes-   600 Subset of directional intra-prediction modes-   601 Angular step between intra-prediction angles in subset-   FIG. 7-   700 Method for intra-predicting a prediction block-   701 Step of selecting an intra-prediction mode-   702 Step of determining a reference sample for a given prediction    sample-   703 Step of applying a filter to the determined reference sample

What is claimed is:
 1. A device, comprising: a non-transitorycomputer-readable storage medium storing instructions; and one or moreprocessors in communication with the medium and upon execution of theinstructions, configured to: select a directional intra-prediction modefrom a set of directional intra-prediction modes, wherein eachdirectional intra-prediction mode of the set of directionalintra-prediction modes corresponds to a different intra-predictionangle; determine, for a prediction sample of a prediction block, areference sample from a set of reference samples based on the selecteddirectional intra-prediction mode; and apply a filter to the determinedreference sample, wherein, within a subset of directionalintra-prediction modes of the set of directional intra-prediction modes,an angular step between the directional intra-prediction modes isdefined in dependence of a filter length of the filter.
 2. The deviceaccording to claim 1, wherein the angular step is larger for a largerfilter length of the filter and smaller for a smaller filter length ofthe filter.
 3. The device according to claim 1, wherein the angular stepis in a range of 2 to 4 degrees.
 4. The device according to claim 1,wherein the filter length of the filter spans 1 to 7 adjacent referencesamples.
 5. The device according to claim 1, wherein the angular step isdenoted Δα and is defined by${\Delta\alpha} = \frac{\arctan N_{f}}{\Delta M_{0}}$ wherein N_(f) isthe filter length of the filter and ΔM₀ is a number of directionalintra-prediction modes in the subset.
 6. The device according to claim1, wherein the filter length of the filter is selected in dependence ofan aspect ratio of the prediction block.
 7. The device according toclaim 1, wherein the filter length of the filter is selected independence of a height or a width of the prediction block.
 8. The deviceaccording to claim 1, wherein the filter performs a smoothing over thedetermined reference sample and one or more adjacent reference samplesaccording to the filter length, when applied to the determined referencesample.
 9. The device according to claim 1, wherein reference samples ofthe set of reference samples are arranged in a row of a video imageadjacently above and above-right the prediction block, or are arrangedin a column of the video image adjacently left and left-under theprediction block.
 10. The device according to claim 1, wherein: thedevice is configured for encoding or decoding a video image; or thedevice is a video encoder or a video decoder.
 11. A method, comprising:selecting a directional intra-prediction mode from a set of directionalintra-prediction modes, wherein each directional intra-prediction modeof the set of directional intra-prediction modes corresponds to adifferent intra-prediction angle; determining, for a prediction sampleof a prediction block, a reference sample from a set of referencesamples based on the selected directional intra-prediction mode; andapplying a filter to the determined reference sample, wherein, within asubset of directional intra-prediction modes of the set of directionalintra-prediction modes, an angular step between the directionalintra-prediction modes is defined in dependence of a filter length ofthe filter.
 12. The method according to claim 11, wherein the angularstep is larger for a larger filter length of the filter and smaller fora smaller filter length of the filter.
 13. The method according to claim11, wherein the angular step is in a range of 2 to 4 degrees.
 14. Themethod according to claim 11, wherein the filter length of the filterspans 1 to 7 adjacent reference samples.
 15. The method according toclaim 11, wherein the angular step is denoted Δα and is defined by${\Delta\alpha} = \frac{\arctan N_{f}}{\Delta M_{0}}$ wherein N_(f) isthe filter length of the filter and ΔM₀ is the number of directionalintra-prediction modes in the subset.
 16. The method according to claim11, wherein the filter length of the filter is selected in dependence ofan aspect ratio of the prediction block.
 17. The method according toclaim 11, wherein the filter length of the filter is selected independence of a height or a width of the prediction block.
 18. Themethod according to claim 11, wherein the filter performs a smoothingover the determined reference sample and one or more adjacent referencesamples according to the filter length, when applied to the determinedreference sample.
 19. The method according to claim 11, whereinreference samples of the set of reference samples are arranged in a rowof a video image adjacently above and above-right the prediction block,or are arranged in a column of the video image adjacently left andleft-under the prediction block.
 20. A non-transitory storage mediumstoring a bitstream, wherein the bitstream is encoded or decoded byexecuting a program stored in the non-transitory storage medium to:select a directional intra-prediction mode from a set of directionalintra-prediction modes, wherein each directional intra-prediction modeof the set of directional intra-prediction modes corresponds to adifferent intra-prediction angle; determine, for a prediction sample ofa prediction block, a reference sample from a set of reference samplesbased on the selected directional intra-prediction mode; and apply afilter to the determined reference sample, wherein, within a subset ofdirectional intra-prediction modes of the set of directionalintra-prediction modes, an angular step between the directionalintra-prediction modes is defined in dependence of a filter length ofthe filter.